The present invention relates to an image forming apparatus of the type transferring a toner image from an image carrier to a sheet-like recording medium by way of an intermediate transfer body.
An electrophotographic image forming apparatus of the type transferring a toner image from a photoconductive drum to a paper or similar sheet-like recording medium by way of an intermediate transfer belt is conventional. It is a common practice with this type of apparatus to set various image forming conditions on the basis of a reference mark or marks provided on the belt. Japanese Patent Laid-Open Publication No. 7-36249, for example, teaches a color image forming apparatus capable of causing an intermediate transfer belt to run at a constant speed by sensing a plurality of marks provided on the belt. Japanese Patent Laid-Open Publication No. 7-325455 discloses a color image forming apparatus in which a reference mark is formed on a photoconductive belt and then transferred to an intermediate transfer belt for the purpose of accurately matching the image areas of the two belts.
Further, Japanese Patent Laid-Open Publication No. 8-101554 proposes a multicolor image forming apparatus including a plurality of sensing means provided on an intermediate transfer belt for sensing changes ion dimension ascribable to temperature or humidity. Moreover, Japanese Patent Laid-Open Publication No. 10-104970 discloses a color image forming apparatus in which a bias is not applied at a reference mark position in order to form an attractive image.
In an image forming process effected with an intermediate transfer belt provided with a single mark (marker hereinafter) as a reference for the start of image formation, every time the belt makes one turn, a single image forming period exists on the belt, and an image transferring period occurs slightly later than the image forming period. This brings about a problem that whatever the image size may be, the printing speed is fixed. Further, when marker sensing means senses the marker of the belt, image formation on a photoconductive drum including in image forming means starts on the elapse of a preselected period of time. Subsequently, as soon as the leading edge of an image on the drum reaches a preselected position in a preselected period of time, the image is transferred from the drum to the belt. In this case, even in a repeat print mode, the next image is not formed until the marker passes the marker sensing means. As a result, the image forming means remains in a stand-by state between the transfer of an image to the belt and the next image forming operation.
Assume that a single marker is provided on the intermediate transfer belt. Then, whenever the apparatus is in a stand-by state, the belt with the single marker remains stationary at the same position. Therefore, in the case where the belt is passed over rollers, the belt is apt to deform. Moreover, because image transfer starts at the same position at all times, deterioration and therefore wear is likely to concentrate at a particular portion of the belt.
Furthermore, to form a color image, the interval between the sensing of the marker and the start of the next image formation must be set beforehand by taking account of a period of time necessary for, e.g., the switching of a developing color. This cannot be done without resorting to a large counter capable of counting a long period of time with accuracy. In addition, irregularities in the movement of the belt accumulate between the sensing of the marker and the start of the next image formation, preventing toner images of different colors from being transferred in accurate register with each other.